First-principles investigations of structural stability and electronic band structure of CH 3 NH 2 BiI 3 for lead-free perovskite solar cell application

Abstract

In this work, first principles density functional theory (DFT) was used to investigate the structural stability and electronic structures of CH 3 NH 2 BiI 3 lead-free perovskite. From the results, CH 3 NH 2 BiI 3 perovskite was predicted to be stable in monoclinic phase (space group P2 1 ) with lattice parameters, a = 8.165 Å, b = 13.194 Å, c = 8.272 Å, and β = 90.03°. The formation enthalpy per formula unit (ΔE) of CH 3 NH 2 BiI 3 was found to be 0.13 eV lower than the total ΔH of CH 3 NH 2 molecule and bulk BiI 3 , indicating its stability with respect to CH 3 NH 2 and BiI 3 . In addition, the chemical potential diagram shows the stable region of CH 3 NH 2 BiI 3 , indicating that CH 3 NH 2 BiI 3 perovskite can be synthesized. From band structure calculations, CH 3 NH 2 BiI 3 has an indirect band gap of 1.58 eV which is comparable to 1.60 eV of CH 3 NH 3 PbI 3 . However, the valence band maximum (VBM) was found to be mainly contributed by N 2p and I 5p, instead of the expected Bi 6s. It is relatively flat compared to the VBM of CH 3 NH 3 PbI 3 , and thus has a larger hole effective mass. However, this theoretical prediction on monoclinic CH 3 NH 2 BiI 3 with enhanced structural stability, synthesizability, and small band gap suggests its capability to be a promising candidate in substituting the lead-based perovskite solar cells.